You need one that somewhat closely matches that voltage, is DC, and can deliver at least that much current, preferably a bit more. Way more current isn't necessarily good in this regard because many wall warts are not regulated and if the load on the wall wart (the current) is way less than its capacity, the voltage output may not sag down to near the specified value, remaining higher than expected.. Granted, unless it's grossly high it probably won't hurt a fan too much other than it running faster.
Drinking: Store bought
Primary: American Amber Ale
Next Up: Dark Lager
Conditioning: Scottish Wee Heavy